1. Field of the Invention
The present invention relates to a transparent thermosensitive recording medium suitable for medical applications, and more particularly to a transparent thermosensitive recording material useful as an image formation sheet capable of producing an image therein with excellent light shielding properties and black coloring performance when viewed on a film viewer called xe2x80x9cSchaukastenxe2x80x9d, and in addition, with excellent image preservation stability.
2. Discussion of Background
Various kinds of recording materials employing the process of thermosensitive coloring reaction are proposed. Such a thermosensitive recording material is generally prepared by coating a mixture of a coloring agent such as a dye, and a color developer on the surface of paper.
In particular, the thermosensitive recording material employing the coloring reaction of a leuco dye has the following advantages:
(a) This type of thermosensitive recording material is prepared by coating a coating liquid comprising a colorless dye precursor and a color developer on a support. When heat is imagewise applied to the thermosensitive recording material, the dye precursor and the color developer dispersed in the form of particles on the support are fused and mixed to cause a coloring reaction. Different from other recording systems such as ink jet printing and thermal image transfer recording, image recording can be achieved by supplying only a thermosensitive recording material. This will be hereinafter referred to as a one-supply system.
(b) When a leuco dye causes a coloring reaction of a black color, the leuco dye shows two broad absorption peaks in the visible region, the two peaks being equal. Therefore, the obtained absorption spectrum becomes flat, so that a black color image can be obtained by use of one kind of leuco dye.
(c) Unlike other recording systems, a recording apparatus for the leuco-type thermosensitive recording material can be made compact by virtue of the fact that the recording can be achieved by the one-supply system, and the recording sensitivity is high.
In light of the above-mentioned advantages, the leuco-type thermosensitive recording material is widely used, for example, as a recording material for facsimile machine, a label for indicating information which is attached to the package of food, and a two-valued character information bearing recording medium for use with delivery management system.
However, the leuco-type thermosensitive recording material has the drawbacks that the background portion tends to cause yellowing and the image portion tends to cause decolorization due to the exposure to heat, light, moisture, and plasticizers. Namely, the preservation stability of the leuco-type thermosensitive recording material is insufficient.
In recent years, disposal of the waste liquid caused by the wet-type image formation process for a silver salt X-ray film has become a serious problem in the medical field. Further, in line with the trend toward the formation of digital image, there is an increasing demand for a dry image formation process using a transparent film capable of easily producing an image therein.
The dry process currently employed in the medical field is divided into the following three systems: (1) light-exposing and heat-fixing system, (2) thermal transfer system, and (3) direct thermosensitive recording system. Among the above-mentioned dry-type systems, the direct thermosensitive recording system (3) attracts special attention because the image formation system is remarkably simple. Namely, image formation can be achieved by the above-mentioned one-supply system, and only a development unit will do for achieving image recording.
Under such circumstances, there is a great demand for a leuco-type thermosensitive recording material not only as the conventional recording medium capable of producing reflection type two-valued character information, but also as a transparent recording medium capable of producing an image with a gray scale, similar to the conventional silver salt film as can be employed in the medical field.
A transmission type image produced in the above-mentioned transparent recording medium for medical applications is expected to satisfy the following requirements: (1) satisfactory light shielding properties and color tone when the image is viewed on the film viewer, (2) sufficient transparency, (3) excellent gradation in color, and (4) high preservation stability. The color gradation (3) of the image depends upon both the recording medium and the recording system therefor. The requirements (1), (2), and (4) are to be controlled by the recording medium itself.
To be more specific, when the leuco-type thermosensitive recording medium is used for medical applications, that is, when the image formed in the transparent thermosensitive recording medium is viewed on the film viewer using transmitted light, there are the following problems.
First of all, when the transmitted light is used, it is difficult for one black coloring leuco dye to produce a black color with satisfactory coloring performance.
FIG. 1 shows absorption spectra of one of the representative leuco dyes capable of producing a black color, using reflecting light and transmitted light. When the transmitted light is used, the black coloring leuco dye exhibits absorption peaks as shown in FIG. 1, and the absorption is decreasing in the near infrared region. The result is that satisfactory black color cannot be produced by use of a single leuco dye. In contrast to this, when the reflected light is used, a satisfactory black color can be produced only by a single leuco dye.
Further, in order to improve diagnostic accuracy, the image formed in the recording material is required to have sufficient light shielding properties when the image is viewed on the film viewer.
The last problem is that not only an image portion with a high transmission density, but also the whole images with different transmission densities are required to have improved preservation stability.
Japanese Patent No. 2773539 discloses a method for obtaining a predetermined black color tone on a reflection type thermosensitive recording material. For adjustment of color tone of the obtained image, a mixture of leuco dyes is removed. To be more specific, a leuco dye capable of producing a black color is used in combination with leuco dyes capable of producing blue and red colors, with the melting points of the blue and red coloring leuco dyes being higher than that of the black coloring leuco dye. When this kind of reflection thermosensitive recording material is applied to the formation of transmission type images, the obtained black color image is not provided with satisfactory color tone, nor sufficient light shielding properties.
Japanese Laid-Open Patent Application No. 08-156430 proposes a method for producing a black color image in a transparent sheet not only by use of a medical film viewer, but also by daylight. However, when a transparent recording sheet is prepared by the coating method using the same leuco dye and color developer as used in the above-mentioned application, the preservation stability of images formed in the recording sheet becomes poor. In addition, the light shielding properties of the image are insufficient when the image is viewed on a medical film viewer.
Accordingly, an object of the present invention is to provide a transparent thermosensitive recording material capable of producing high quality images, in particular, medical images for diagnostic use which are viewed on a film viewer, that is, xe2x80x9cSchaukastenxe2x80x9d, with sufficient light shielding properties, excellent black color tone, and high preservation stability.
The above-mentioned object of the present invention can be achieved by a transparent thermosensitive recording material capable of producing an image therein, comprising a transparent support and a thermosensitive recording layer formed thereon comprising a colorless or light-colored leuco dye, a color developer capable of inducing coloring formation in the leuco dye, and a binder resin, wherein the image formed in the recording material exhibits an absorbance of 3.0 or more at 610 nm when the image has a maximum transmission density, and the ratio of the absorbance at 700 nm to the absorbance at 610 nm is in a range of 0.1 to 0.3 within the region from a transmission density of the image of 1.0 to the maximum transmission density thereof.